The present invention relates to a hydraulic brake system for a road vehicle with single-axle drive, and, more particularly, to a brake system which uses a pressure modulator to obtain greater sensitivity.
A hydraulic brake system is shown in DE 38 12 830 C2 for a rear-axle drive road vehicle which is also equipped with an antilocking brake system which operates on the front-axle brake circuit in accordance with the known "pump-back" principle and on the brake circuit of driven vehicle wheels in accordance with a pressure modulation principle by changing the volume of a modulation chamber of a pressure modulator connected to the rear axle brake circuit.
The known pressure modulator is also used to actuate a traction control device for a wheel tending to spin, which device operates on a retarding principle, i.e. retarding the spinning wheel by activating its wheel brake. The modulation chamber is connected to and can be shut off or disconnected from the rear wheel brakes by brake pressure control valves individually associated with the rear wheel brakes of the vehicle. These valves are used to control the pressure inlet and pressure outlet. The modulation chamber has pressure-tight and displaceable separation from a drive pressure space of the pressure modulator via a modulator piston.
When the known drive pressure space is subjected to the high outlet pressure of an auxiliary pressure source, the modulator piston can be displaced to reduce the volume of the modulator chamber in order to achieve a build-up of brake pressure to effect the traction control function. The piston can be displaced by pressure relief of this drive pressure space to increase the volume of the modulation chamber to effect the pressure reduction function in antilocking control operation.
In normal braking operation, the brake pressure is built up by actuation of the brake unit, whose outlet pressure is fed to the wheel brakes via (1) a solenoid valve designed as a 2/2-way valve whose basic position 0 is its flow position and whose excited position I is its shut-off position, (2) the modulation chamber and (3) brake pressure control valves by virtue of which the modulation chamber is in communicating connection with the wheel brakes. In normal braking operation, the modulator piston is held in a position corresponding to the minimum volume of the modulation chamber by connection of its drive pressure space to the high pressure outlet of the auxiliary pressure source.
The pressure modulator is configured such that a single piston stroke of the modulator piston, by way of which the modulator piston moves from its end position corresponding to the minimum volume of the modulation chamber into its end position corresponding to the maximum volume of the modulation chamber, is sufficient, even if a brake fluid quantity corresponding to maximum brake pressure has previously been displaced into the wheel brake cylinders, to effect complete removal of pressure. If the piston had been in its position corresponding to the maximum volume of the modulation chamber and was to be displaced into its end position corresponding to the minimum volume of the modulation chamber in traction control operation, the single stroke of the modulator piston is sufficient to build up in the wheel brakes the maximum brake pressure which could be built up by actuation of the brake unit.
In an antilocking brake system operation, the solenoid valve is driven into its shut-off position so that only the wheel brakes are connected, via the brake pressure control valves, to the modulation chamber of the pressure modulator. The brake unit is shut-off by the solenoid valve from the pressure modulator and hence also from the wheel brakes in traction control operation. In order for both antilocking and traction control to be able, if required, to take place in correct phase, i.e. the antilocking control with a pressure reduction phase and the traction control with a brake pressure build-up phase, the modulator piston is held in an intermediate position, e.g. the central position, between its possible end positions. This intermediate position is monitored by an electronic position signal generator and maintained by the electrical triggering of solenoid valves which provide the connection between the drive pressure space and the pressure outlet of the auxiliary pressure source or its pressurized reservoir.
Starting from this central position of the piston, only half the modulation chamber volume, so to speak, is available initially, however, for a pressure reduction phase initiating an antilocking control cycle. As a result, in a situation demanding antilocking control in which the maximum brake pressure in the wheel brake cylinders has been previously built up, an effective antilocking control nevertheless requires a substantial brake pressure reduction to, for example, 10% of this maximum value. This pressure reduction due to the end position corresponding to the maximum volume of the modulation chamber from the intermediate position of the piston only is in sufficient for such a pressure reduction.
The foregoing is similarly applicable to traction control operation when the latter demands the build-up of a high brake pressure in the wheel brakes of both driven vehicle wheels. The average value of such pressure is higher than half the maximum pressure which can be built up in the wheel brakes. In order, nevertheless, to permit a complete reduction of pressure at high brake pressures in the course of an antilocking control, a pump-back mode of operation for the pressure modulator is possible in which the pressure modulator pumps back into the brake unit brake fluid accepted previously in a pressure reduction stroke with the solenoid valve open.
Brake pressure reduction in several partial strokes is also possible in a analogous manner in traction control operation. In the known brake system, an electronic force sensor integrated in the modulator piston is provided in addition to the electronic position signal generator recording the piston position. The output of this force sensor is a measure of the brake pressure present in the modulation chamber and hence also in the wheel brakes. In the known brake system, information on the functional state of elements affecting safety in the brake system is gained from the output signal of these two signal generators characteristic of position and pressure, and possibilities are also provided for an electronically controlled pressure metering or pressure reduction to match the particular control situation.
At least the following disadvantages derive from the structural and functional properties of the above-described brake system. Both the antilocking control and the traction control operate sluggishly when, in antilocking operation, high magnitude pressure reductions are necessary and, in traction control operation, when high brake pressures are necessary. This is because there are dead periods associated with the pump-back or replenishing operating phases, and the brake pressure in a wheel brake subject to the control system is necessarily kept constant during these periods.
Frequent control of a functional control valve necessary for positioning the modulator piston in the central position as the initial position for both antilocking and traction control operation increases the wear on this valve and can cause additional susceptibility to faults.
An object of the present invention is, therefore, to improve a known brake system such that, despite simple structure and high reliability, an improvement is achieved in the sensitivity of the possible types of control.
This object has been achieved according to the present invention by holding the piston by a return spring in its position corresponding to the maximum volume of the modulation chamber in the non-actuated condition of the brake system. The modulation chamber is connected to the pressure outlet of the brake unit associated with the brake circuit of the driven vehicle wheels. As soon as the brake system is actuated or the traction control responds, the modulation chamber is shut off from the pressure outlet. On each braking action, the brake pressure build-up in the brake circuit of the driven vehicle wheels takes place via displacement of the modulator piston. An electronic braking force distribution control device operates with monitoring of the piston position and/or the brake circuit of the driven vehicle wheels and with recording of the brake pressure in the brake circuit of the non-driven vehicle wheels or of the position correlated with the non-driven wheels of a functional element of the brake unit. The control device maintains the brake pressure in the brake circuit of the driven wheels, during normal braking operation, in a defined relationship with the brake-pressure fed into the circuit of the non-driven vehicle wheels.
The pressure modulator is used for building up the brake pressure in the brake circuit of the driven vehicle wheels even during normal braking, i.e. braking not subject to a control system, in combination with dimensioning of the pressure modulator. Thus, the maximum brake pressure can be built up in a single stroke of its piston, with the position corresponding to the maximum volume of the modulation chamber being provided as the basic position of the modulator piston.
The piston is in a position, whenever the antilocking control system responds, in which the brake pressure previously connected to the wheel brakes of the driven vehicle wheels can also be brought down completely by a rapid pump-back stroke of the piston.
Similarly, the connection of a maximum brake pressure to one or more of the wheel brake subject to the control system is also possible in a pressure build-up stroke of the piston when the traction control responds. In addition, the brake system according to the present invention also permits control of the brake force distribution, in particular such that an increased contribution of the rear wheel brakes to the total braking force available can be used in the partial braking range.
Another embodiment of a brake system according to the present invention can be effected directly by retrofitting a vehicle equipped with a standard system operating on the pump-back principle with a pressure modulator suitable for traction control and electronic braking force distribution. If the standard antilocking system has brake pressure control valves individually associated with the driven vehicle wheels, no additional solenoid valves are then required. In the situation where the system is configured for control on the "select-low" principle with respect to the driven vehicle axle, usually the rear axle, only one common 3/3-way solenoid valve is needed for the brake pressure control on both wheel brakes, with a 2/2-way solenoid valve connected between the 3/3-way solenoid valve and the wheel brakes. This latter configuration is however, only a relatively small extra requirement.
A pressure modulator having a stepped piston with a smaller diameter step forming an axially movable boundary of the modulation chamber and whose larger diameter step forms an axially movable boundary of the control pressure space of the pressure modulator makes it possible to adapt the pressure modulator, as required, to the outlet pressure level of an existing auxiliary pressure source on the vehicle.
The use of a standard pump-back device configured for an brake system operating on both brake circuits in accordance with the "pump-back" principle overall brake system structure. Because the auxiliary pressure source operates at an outlet pressure level corresponding at least to the brake pressure level, it permits a simple arrangement of the pressure modulator as a pressure transformer and also operation of the control circuit of the pressure modulator with brake fluid as the working medium. As a result, otherwise necessary measures for separating the media between the drive circuit of the pressure modulator and its brake pressure outlet circuit become unnecessary.
It has also been found advantageous to provide safety measures in the form of a pressure limiting valve and opening and closing of a hydraulic control parts under valve control, and for providing pressure feed to the control pressure space of the pressure modulator provided with a throttle, particularly where the auxiliary pressure source can operate at a very high outlet pressure level.
A valve which shuts off the modulation chamber of the pressure modulator from the brake unit in traction control and electronic braking force distribution operation as a solenoid valve has also been found to have safety advantages.
The determination of the brake pressure connected, via the pressure modulator, to the wheel brakes of the driven vehicle wheels by monitoring the position of the modulator piston has the advantage that the output signal, from an appropriate position signal generator, which is characteristic of position, is also a measure of the brake pressure when the wheel brakes are shut off from the modulation chamber and the high outlet pressure of the auxiliary pressure source is connected to the drive pressure space of the pressure modulator. Consequently, the existing pressure in the modulation chamber is substantially higher than that in the wheel brakes.